In recent years, there are proposed many semiconductor memory devices in which memory cells are disposed three-dimensionally in order to increase a degree of integration of memory.
One conventional semiconductor memory device in which memory cells are disposed three-dimensionally uses a transistor with a cylindrical column type structure (refer to J.P. 2007-266143). The semiconductor memory device using the transistor with a cylindrical column type structure is provided with a stacked conductive layer stacked in multiple layers and configured to form a gate electrode, and a pillar-shaped columnar semiconductor. The columnar semiconductor functions as a channel (body) portion of the transistor. A vicinity of the columnar semiconductor is provided with a memory gate insulating layer capable of storing a charge. A configuration including these stacked conductive layers, the columnar semiconductor, and the memory gate insulating layer is called a memory string.
To achieve conduction with a peripheral circuit in a semiconductor memory device having the above-described memory string, a contact plug is formed extending in a stacking direction (direction perpendicular to a substrate) from the stacked conductive layer. Generally, the contact plug is formed by the following process. That is, first, an interlayer insulating layer is formed so as to cover the stacked conductive layer. Subsequently, the interlayer insulating layer is penetrated so as to reach an upper surface of the respective stacked conductive layers, thereby forming a hole. Then, a conductive layer is deposited so as to fill the hole, thereby forming the contact plug.
However, the above-described manufacturing process causes the contact plug to be formed in a tapered shape in which a diameter of the contact plug becomes smaller from an upper end to a lower end thereof. Accordingly, the lower the layer in which the contact plug has its lower end, the more difficult it becomes, during manufacture, to fill the hole with the conductive layer. In addition, the lower the layer in which the contact plug has its lower end, the more difficult it becomes, during manufacture, to form the hole completely penetrating the interlayer insulating layer. Moreover, the lower the layer in which the contact plug has its lower end, the smaller becomes a contact area with the stacked conductive layer. It is difficult to manufacture a nonvolatile semiconductor memory device having a high reliability using the above-described manufacturing process.